This invention relates to an image printing system for printing digital image data sent from a computer or a signal generator, and more particularly relates to an image printing system adapted to be ready to receive image data in response to a print command from a computer or a signal generator and ready to receive a command upon receipt of a designated number of data.
In the prior art printer system, 1 byte followed by an escape code is recognized as a command. When an image is printed, the number of print lines or print dots is specified by a command from a host computer. After this, another command is entered for a graphic code, i.e., the image data reception state. Then, the command reception mode is automatically restored after the specified image data have been received.
Here, the image data of 32 gradations or more comprise data of 1 byte per each picture element so that data of three colors have 40 mega p bytes if the A3 size paper having 12 picture elements/mm. If such a large amount of data are transferred from the host computer to the printer, there may be caused by external noises a difference between the number of data sent actually by the host computer and the number of data received by the printer. Especially in the case that the interface has no parity checking function such as Centronics and the length of the cable connected between the host computer and the image printer extends a few meter this phenomenon is frequently caused. In the case of Centronics, the number of data is increased if noise enters the strobe signal at the host side. If, on the other hand, noise enters the ready (or busy) signal at the printer side and then the ready signal is masked, the data become short. This phenomenon arises probabilistically with the increase in the circumstances of many noises. FIG. 3 shows a timing chart used to explain why there arise a difference between the number of data actually received by the host computer and the number of data received by the printer. The timing chart in the normal operation is shown in the A interval. The timing chart in the state of the excess data is shown in the C interval. Noise signals enter the strobe signals sent from the host computer and a false strobe signal is generated. The false strobe signal (X) causes the data D2 to be received two times. As a result, excess data are caused. The timing chart in the state of short data is shown in the B interval. The large amplitude and low frequency noise caused by such as an electro-magnetic induction causes a fluctuation of the ground level (Y.sub.0) of the busy signals. The host computer detects a busy signal (Y.sub.1) as a low level signal in error and sends the next image data (D.sub.5). As a result, the printer can not receive the image data (D.sub.5).
If the data become excess, the data left after reception of a predetermined number of data are judged as the command and such raises problems that the printer may accomplish abnormal operations and that the printing operation is interrupted by the error state.
If the data number is short, on the contrary, the printer continues to await sufficient image data so that it is interrupted in the unprintable state or prints a subsequent command as the image. As a result, the printer may fail to perform the operation to be accomplished intrinsically by the command or may perform an abnormal operation or make a stop.